In general, how is the final setting of concrete affected by adding SCMs?

• A. It remains the same
• B. It accelerates
• C. It delays
• D. It becomes unpredictable

Answer: (A)

The effect hinges on how SCMs change the chemical reactions in concrete. Replacing part of the cement with materials like fly ash, slag, or silica fume generally slows the rate of cement hydration because there is less cement reacting quickly at the start. The pozzolanic or latent hydraulic reactions of these materials tend to occur later, which delays the stiffening process and, therefore, the final set. In practice, this means concrete often takes longer to reach its final set when SCMs are used, especially at typical curing temperatures and dosages. There are exceptions depending on the specific SCM, dosage, and curing conditions, but the general trend is a delay rather than no change.